1. Field of the Invention
This invention relates to a continuous production process of a propylene-ethylene block copolymer, using a highly active catalyst, and further it relates to a process for producing a copolymer having very well-balanced qualities of impact strength, stiffness, processability, etc. with a good productivity.
2. Description of the Related Art
Crystalline polypropylene produced using a stereospecific catalyst has superior properties such as stiffness, heat resistance, etc., but on the other hand, it has a problem that its impact strength, particularly, low temperature impact strength, is low; hence its use application range has been practically restricted. Thus, in order to solve the problem, a number of copolymers of propylene with ethylene or other olefins have been proposed. In the case of propylene-ethylene block copolymerization process, the low temperature impact strength can be improved to a large extent without damaging the stiffness, heat-resistance, etc. which are superior characteristics of polypropylene, but, on the other hand, a problem has been raised in the aspect of production and quality specific of the propylene-ethylene copolymerization process. Namely, in the case of the production of this propylene-ethylene block copolymer by way of batch polymerization process, the yield of the polymer per unit time and per unit polymerization vessel is lower than that in the case of continuous polymerization process and hence the cost becomes higher. On the other hand, in the case of a multistage, continuous polymerization process, the retention times of the respective catalyst particles in the respective stage polymerization vessels, respectively, form a distribution (which is considered to be close to the distribution in a completely mixing vessel); hence the resulting aggregates of polymer particles have distibutions in the content ratio of the polypropylene portion (PP portion: a portion containing propylene in a large quantity i.e. a portion prepared in the polymerization step (I)) and the content ratio of the propylene-ethylene random copolymer portion (RC portion: a portion containing ethylene in a large quantity i.e. a portion prepared in the polymerization step (II)); hence a problem occurs in the aspect of production and quality, originated from the non-uniformity of the distributions. Thus, a number of proposals for overcoming such a drawback of the continuous polymerization process have been made.
For example, Japanese patent application laid-open Nos. Sho 58-49716, Sho 55-116716, Sho 58-69215, etc. have proposed a process of classifying the slurry having left the above-mentioned polymerization step (I) by means of a cyclone and again returning fine particles to the polymerization step (I), but since such a classification by way of catalyst particle size does not always accord with the distribution of retention time, improvement in the non-uniformity is insufficient.
Japanese patent application laid-open Nos. Sho 57-195718, Sho 58-29811, etc. have disclosed a process of intermittently carrying out the feed of catalyst into the polymerization vessel and removal of slurry therefrom to thereby reduce the quantity of the catalyst entering the polymerization step (II) during a short retention time, but this has raised a problem that the polymerization reaction is unstable.
Further, there have been proposed some processes as in the process of the present invention, wherein the slurry having left the polymerization step (I) is treated with an electron-donating compound or the like to thereby selectively inactivate the catalyst having left the stage within a short retention time (short-cut-pass catalyst). For example, according to the use of electron-donating compounds proposed in Japanese patent application laid-open Nos. Sho 58-32615, Sho 57-174311, Sho 57-147508, Sho 57-145115, Sho 55-115417, etc., the effectiveness has been insufficient for achieving the continuous production process of propylene-ethylene block copolymer having physical properties corresponding to those according to batch type polymerization process. Further, according to a process proposed in Japanese patent application laid-open No. Sho 62-116618, a polymer having physical properties corresponding to those obtained according to batch type polymerization process has been obtained, but since a large reduction in the activity during the polymerization step (II) occurs, there has been raised a problem that the quantity polymerized at the polymerization step (II) cannot be raised.
In the present invention, the present inventors have this time found a compound having such an effectiveness far larger than that of so far known compounds, that the short cut pass catalyst having passed through the above polymerization step (I) while keeping a far shorter retention time of catalyst than the average retention time is selectively inactivated by the compound to thereby suppress the polymerization reaction at the above polymerization step (II), and prevent formation of polymer particles having a higher ratio of RC portion at this polymerization stage. As a result, by mixing the above found compound with the polymer formed at the polymerization step (I), the above problems of the prior art could have been solved.
As apparent from the foregoing, the object of the present invention is to provide a continuous production process of ethylene-propylene copolymer having solved the problems of the prior art by employing a specified polymerization process and a specified compound, to thereby obtain a superior copolymer having well-balanced properties of impact resistance, stiffness, processing characteristics, etc. with a good yield.